1. Field of the Invention
The present invention relates to a wireless transmission repeater system and wireless equipment for use in the system. More particularly, the invention relates to a wireless transmission repeater system in which a plurality of repeaters are disposed between a transmitter and a receiver in a wireless transmission repeater system such as a mobile communication system, and wireless equipment used for the system.
2. Description of the Related Art
In a conventional mobile communication system, to improve reception sensitivity of a receiver of a radio signal transmitted via a transmission antenna from a transmitter, selective diversity reception of employing two reception antennas and using a reception signal from the antenna of a higher signal-to-noise (S/N) ratio, and synthetic diversity reception of adding signals from two reception antennas in accordance with the S/N ratio are known. In a 3GPP (Third Generation Partnership Project) as one of standardization organizations of third-generation mobile communication standards, a transmission diversity technique of improving reception sensitivity of a receiver by making a path (propagation path) of a radio signal to the receiver insusceptible to the influence of obstacles from the viewpoint of probability by transmitting the same signal from two transmission antennas is known.
A standardization organization (3GPP2) examining a communication system different from the 3GPP has proposed a communication system (MIMO: Multiple Input Multiple Output) such that, as shown in FIG. 1, a transmitter 101 transmits transmission data so as to be distributed to M pieces of antennas, radio signals arriving via transmission paths H are received via N antennas by a receiver 102, and M transmission signals are obtained from N reception signals by signal processing, thereby obtaining received data.
The principle of the MIMO system will be described. When M transmission signal vectors distributed by the transmitter 101 are set as s=(s1, s2, . . . , sM), a signal is transmitted from a transmission antenna “i” is multiplied by hji and a resultant signal is received by a reception antenna “j”, a reception signal xj by the receiving antenna “j” is expressed by the following expression (1).xj={Σ(hji×si)}+vj {i=1 to M}  (1)where vj denotes noise which occurs at the reception antenna “j” in the receiver 102. When a matrix H of N rows and M columns using hji as an element, reception signal vector x=(x1, x2, . . . , xN), and noise vector v=(v1, v2, . . . , vN) which occurs in the receiver 102 are used, the expression (1) can be expressed as the following expression (2).x=Hs+v  (2)
When the propagation path matrix H and the noise vector v in the expression (2) are known, a transmission signal “s” can be obtained from the reception signal “x”. However, the propagation state between the transmitter 101 and the receiver 102 changes according to time and a place of communication, so that the propagation path matrix H is not determined unconditionally. Consequently, a training signal is multiplexed on a transmission signal, the training signal is received by the receiver 102, and an action matrix W for obtaining the transmission signal from the received signal is calculated. By obtaining the action matrix W, the state of the propagation path matrix H and the noise vector “v” is learned by training. By using the reception signal “x” which is received after the training period and the action matrix W, an estimation value s′ of the transmission signal can be expressed by the following equation (3).s′=WTx  (3)
According to the MIMO system, data of an amount which is ideally larger by the number (M times) of transmission antennas as compared with the system of sending transmission data from a single antenna can be transmitted. Consequently, the MIMO system is expected as a system of a large communication capacity per a radio frequency band (high frequency use efficiency).
As shown in FIG. 2A, in the case where many buildings exist as in cities, radio signals from the transmitter 101 are reflected by many buildings and arrives at the receiver 102 via various paths. Such propagation paths will be called a multipath transmission environment. As shown in FIG. 2B, transmission paths in which no obstacles and buildings by which a signal is reflected exist between the transmitter 101 and the receiver 102 will be called an insight transmission environment. FIG. 2C is a graph showing the characteristics of the capacity of communication path and the number of transmission/reception antennas in the MIMO system with respect to the multipath transmission environment and the insight transmission environment. It is understood from FIG. 2C that the capacity of the communication path increases in proportional to increase in the number of transmission/reception antennas in the multipath transmission environment, whereas the capacity of the communication path in the insight transmission environment is smaller than that of the multipath transmission environment and, even when the number of transmission/reception antennas increases, the capacity of the communication path does not increase. In the example shown in FIGS. 2A to 2C, when the number of transmission antennas of the transmitter 101 is 3 and the number of reception antennas of the receiver 102 is 3, reception signals x1, x2, and x3 can be expressed as the following expressions (4), (5), and (6), respectively.x1=h11s1+h12s2+h13s3+v1  (4)x2=h21s1+h22s2+h23s3+v2  (5)x3=h31s1+h32s2+h33s3+v3  (6)
In the multipath transmission environment, since a path from a transmission antenna 201a to a reception antenna 202a and a path from a transmission antenna 201b to the reception antenna 202a are different from each other, the elements h11 and h12 in the propagation path matrix have different values. On the other hand, in the insight transmission environment, as compared with the distance between the transmission antennas 201a and 201b, the distance between the transmission antenna 201a and the reception antenna 202a and the distance between the transmission antenna 201b and the reception antenna 202a are sufficiently longer. Consequently, the transmission antennas 201a and 201b seem to be almost in the same position from the reception antenna 202a, so that the propagation path matrix elements h11 and h12 have similar values. For a reason similar to the above, h11 and h13 have similar values, and h11 and h21 have similar values, so that it becomes difficult to separate the transmission signals s1, s2 and s3 from the expressions (4), (5), and (6).
Accordingly, it can be said that the MIMO system has the better communication path capacity characteristic in the multipath transmission environment as compared with the insight transmission environment. On the other hand, also in the insight transmission environment, it is desired to provide the MIMO system of a larger communication path capacity.